1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to an active matrix organic electroluminescent display (OELD) device and method of fabricating an OELD device.
2. Discussion of the Related Art
Organic electroluminescent display devices include a cathode electrode to inject electrons, an anode electrode to inject holes, and an organic electroluminescent layer between the two electrodes. An organic electroluminescent diode has a multi-layer structure of organic thin films provided between the anode electrode and the cathode electrode. When a forward current is applied to the organic electroluminescent diode, electron-hole pairs (often referred to as excitons) combine in the organic electroluminescent layer as a result of a P-N junction between the anode electrode and the cathode electrode. The electron-hole pairs have a lower energy when combined as compared to when they are separated. The resultant energy gap between the combined and separated electron-hole pairs is converted into light by an organic electroluminescent element. In other words, the organic electroluminescent layer emits the generated energy due to the recombination of electrons and holes in response to an applied current.
As a result of the above-described principles, the organic electroluminescent display devices do not need an additional light source as compared to related art liquid crystal display devices. Moreover, the electroluminescent display devices are thin, light weight, and are very energy efficient. As a result, the organic electroluminescent display devices have excellent advantages when displaying images, for example, low power consumption, high brightness, and a short response time. Because of these advantageous characteristics, the organic electroluminescent display devices are regarded as a promising candidate to be implemented in the various next-generation consumer electronic appliances, such as mobile communication devices, CNS (car navigation system), PDAs (personal digital assistances), camcorders, and palm PCs. Also, because the fabrication of such the organic electroluminescent display devices is a relatively simple process, production cost for the organic electroluminescent display devices is lower than that of the related art liquid crystal display devices.
The organic electroluminescent display devices may be provided in either a passive matrix type arrangement or an active matrix type arrangement. The passive matrix type has a simple structure and fabrication process, but has a high power consumption than the active matrix type. Furthermore, because the structure of passive matrix organic electroluminescent display devices limits the display size, it is difficult to adapt the passive matrix type to large sized devices. Moreover, the aperture ratio of the passive matrix type decreases as the bus lines increases. In contrast, active matrix type organic electroluminescent display devices provide a higher display quality with higher luminosity than that of the passive matrix type.
FIG. 1 is a schematic cross-sectional view illustrating an active matrix type organic electroluminescent display device according to the related art arrangement. As shown in FIG. 1, an organic electroluminescent display device 10 includes first and second substrates 12 and 28, which are attached to each other by a sealant 26. On the first substrate 12, a plurality of thin film transistors (TFTs) T and array portions 14 are formed. Each of the TFTs T corresponds to each pixel region P. A first electrode (i.e., an anode electrode) 16, an organic electroluminescent layer 18 and a second electrode (i.e., a cathode electrode) 20 are sequentially formed on the array portion 14. At this point, the organic electroluminescent layer 18 emits light corresponding to red (R), green (G) or blue (B) color in each pixel P. In particular, to show color images, organic materials emitting the R, G and B colors are disposed respectively in each pixel P.
As additionally shown in FIG. 1, the second substrate 28, which is attached to the first substrate 12 by the sealant 26, includes a moisture absorbent (i.e., desiccant) 22 on the rear surface thereof. The moisture absorbent 22 absorbs the moisture that may exist in the cell gap between the first and second substrates 12 and 28. When disposing the moisture absorbent 22 in the second substrate 28, a portion of the second substrate 28 is etched to form a dent. Thereafter, the moisture absorbent 22 that may be a powder type is disposed into this dent, and then, a sealing tape 25 is put on the second substrate 28 to fix the powder-type moisture absorbent 22 in the dent.
FIG. 2 is an equivalent circuit diagram illustrating a pixel of the organic electroluminescent display device according to the related art arrangement. As shown in FIG. 2, a gate line 36 is disposed in a transverse (i.e., horizontal) direction and a data line 49 is disposed in a longitudinal (i.e., vertical) direction substantially perpendicular to the gate line 36. A switching thin film transistor (switching TFT) TS is disposed near a crossing of the gate and data lines 36 and 49 and a driving thin film transistor (driving TFT) TD is disposed electrically connecting to the switching thin film transistor TS and a power line 62. In addition, the driving TFT TD is electrically connected to an organic electroluminescent diode E. A storage capacitor Cst is disposed between a driving source 52 and a driving gate 34 of the driving TFT TD. The storage capacitor Cst is also connected to a switching drain of the switching TFT TS and the power line 62. A switching source of the switching TFT TS is connected to the data line 49, and a driving source 52 of the driving TFT TD is connected to the power line 62. A switching gate 32 of the switching TFT TS is connected to the gate line 36. The organic electroluminescent diode E comprises a first electrode, an organic electroluminescent layer and a second electrode, as described in FIG. 1. The first electrode of the organic electroluminescent diode E electrically contacts with a driving drain 54 of the driving TFT TD, the organic electroluminescent layer is disposed on the first electrode, and the second electrode is disposed on the organic electroluminescent layer.
Now, an operation of the organic electroluminescent display device will be briefly explained with reference to FIG. 2. When a gate signal is applied to the switching gate 32 of the switching TFT TS from the gate line 36, a data current signal flowing through the data line 49 is converted into a voltage signal by the switching TFT TS to be applied to the driving gate 34 of the driving TFT TD. Thereafter, the driving TFT TD is operated and determines a current level that flows into the organic electroluminescent diode E. As a result, the organic electroluminescent diode E can display a gray scale between black and white.
The voltage signal is also applied to the storage capacitor Cst such that a charge is stored in the storage capacitor Cst. The charge stored in the storage capacitor Cst maintains the voltage of the voltage signal on the driving gate 34. Thus, although the switching TFT TS is turned off, the current level flowing to the organic electroluminescent diode E remains constant until the next voltage signal is applied.
Meanwhile, the switching and driving TFTs TS and TD may include either a polycrystalline silicon layer or an amorphous silicon layer. When the TFTs TS and TD include an amorphous silicon layer, fabrication of the TFTs TS and TD is much simpler as compared to TFTs TS and TD that include a polycrystalline silicon layer.
FIG. 3 is a schematic plan view of an active matrix organic electroluminescent display device having an amorphous silicon layer according to the related art. As shown in FIG. 3, the active matrix organic light emitting diode device includes, for example, inverted staggered type thin film transistors that functions as a bottom emission type.
A gate line 36 intersects a data line 49 and a power line 62, in which the data line and the data and power lines 49 and 62 respectively are spaced apart from each other. A pixel region defined by the gate line 36 and intersections of the data and power supply lines 49 and 62. A switching thin film transistor (TFT) TS is disposed adjacent to a position where the gate line 36 and the data line 49 cross each other. A driving thin film transistor (TFT) TD is disposed next to the power line 62 and in the pixel region. The driving TFT TD has a larger size than the switching TFT TS, and therefore, the driving TFT TD occupies a relatively large space of the pixel region.
The switching TFT TS includes a switching gate 32 extending from the gate line 36, a switching source 48 extending from the data line 49, a switching drain 50 spaced apart from the switching source 48, and a switching active layer 56a above the switching gate electrode 32. The switching active layer 56a is formed of amorphous silicon and has an island shape.
The driving TFT TD is connected to the switching TFT TS and the power line 62. The driving TFT TD includes a driving gate 34, a driving source 52, a driving drain and a driving active layer 58a. The driving gate 34 is connected with the switching drain 50 and elongates along side of the power line 62. The driving active layer 58a is formed of amorphous silicon and has a long island shape. Additionally, the driving active layer 58a also elongates along side of the power line 62 while also corresponding and overlapping the driving gate 34. The driving source and drain 52 and 54 overlap side portions of the driving gate 34. The driving active layer 58a having an island shape is disposed above the driving gate 34 between the driving source and drain 52 and 54.
As also shown in FIG. 3, the power line 62 has a protrusion extending to the driving source 50 and electrically communicates with the driving source 50 through the protrusion. A first electrode 66 of the organic electroluminescent diode is disposed in the pixel region and connected with the driving drain 54.
The driving thin film transistor TD needs to have an ability to operate and drive the organic electroluminescent diode. Thus, a channel of the driving thin film transistor TD should have a large channel width W and a short channel length L such that the ratio of width W and length L should be large enough. Thus, the driving thin film transistor TD can supply sufficient current to the organic electroluminescent diode to operate and to drive the organic electroluminescent diode.
FIGS. 4A and 4B are cross sectional views taken along lines IVa-IVa and IVb-IVb of FIG. 3, illustrating the switching thin film transistor and the driving thin film transistor, respectively.
In FIGS. 4A and 4B, the switching gate 32 and the driving gate 34 are formed on a substrate 30. Although not shown in FIGS. 4A and 4B, but shown in FIG. 3, the gate line 36 is also formed on the substrate 30. As described before, the driving gate 34 is larger than the switching gate 32 and occupies a large portion of the pixel region. A gate insulating layer 38 is formed on the substrate to cover the driving and switching gates 32 and 34 and the gate line 36. The gate insulating layer 38 has a contact hole that exposes one end of the driving gate 34. A switching semiconductor layer 56 and a driving semiconductor layer 58 are formed on the gate insulating layer 38, respectively, above the switching gate 32 and above the driving gate 34. The switching semiconductor layer 56 comprises a switching active layer 56a of undoped amorphous silicon and a switching ohmic contact layer 56b of doped amorphous silicon. The driving semiconductor layer 58 also comprises a driving active layer 58a of undoped amorphous silicon and a driving ohmic contact layer 58b of doped amorphous silicon. As shown in FIG. 3, the driving semiconductor layer 58 is larger than the switching semiconductor layer 56 enough to overlap the driving gate 34. The switching source and drain 48 and 50 are formed spaced apart from each other and contact the switching ohmic contact layer 56b, and the driving source and drain 52 and 54 are formed spaced apart from each other and contact the diving ohmic contact layer 58b. The switching drain 50 also electrically contacts the driving gate 34 within the contact hole defined through the gate insulating layer 38. The data line 49 is also formed on the gate insulating layer 38 integrated with the switching source 48 and disposed perpendicularly intersecting the gate line 36, as shown in FIGS. 3 and 4A. Accordingly, the switching thin film transistor TS and the driving thin film transistor TD are formed.
A first passivation layer 60 is formed over the entire of the substrate 30 to cover the switching thin film transistor TS and the driving thin film transistor TD. The first passivation layer 60 has a contact hole that exposes a portion of the driving source 52. Then, the power line 62 is formed on the first passivation layer 60 and contacts the driving source 52 within the contact hole defined through the first passivation layer 60, as shown in FIG. 4B. The power line 62 is spaced apart from the data line 49 and perpendicularly intersects the gate line 36, as shown in FIG. 3, thereby defining the pixel region with the gate and data lines 36 and 49. A second passivation layer 64 is formed over the entire surface of the first passivation layer 60 to cover the power line 62. The first and second passivation layers 60 and 64 have a contact hole that exposes a portion of the driving drain 54. The first electrode 66 of the organic electroluminescent diode is formed on the second passivation layer 64 and electrically contacts the driving drain 54 within the contact hole defined through the first and second passivation layers 60 and 64. The first electrode 66 is disposed in the pixel region as shown in FIG. 3.
In the related art shown in FIGS. 3 and 4A-4B, the driving active layer 58a has a wide channel width and a short channel length, accordingly the driving thin film transistor TD occupies a large amount of the pixel region. Thus, an aperture ratio of the bottom emission type organic electroluminescent display device is decreased. Furthermore, since a large amount of current flows through the driving thin film transistor TD, current stress may be caused in the driving thin film transistor TD, thereby damaging the driving thin film transistor TD. Especially, when the DC bias is continuously applied to the driving thin film transistor TD, the electrical properties of the driving thin film transistors TD deteriorates and eventually malfunctions. Furthermore, since only one driving thin film transistor TD is adopted to operate the organic electroluminescent diode in a pixel, the deterioration of the driving thin film transistor TD may be accelerated. Accordingly, the active matrix organic electroluminescent display devices having the above-mentioned driving thin film transistor may show an residual image phenomenon, resulting in poor display quality. Moreover, when the driving thin film transistor is deteriorated and malfunctioned by the electrical stress, a point defect occurs in the pixel.
As above described, the organic electroluminescent display devices of FIGS. 1-3 and 4A-4B is a bottom emission type according to an emission direction of light used for displaying images. The bottom emission type organic electroluminescent display device has the advantage of high encapsulation stability. However, the bottom emission type organic electroluminescent display devices are ineffective as high-resolution devices since the disposition of the thin film transistors and the storage capacitor formed on the substrate results in a poor aperture ratio. In contrast to the bottom emission type, a top emission type organic electroluminescent display device has a higher aperture ratio because they have simpler circuit layouts that make it possible to direct the emitted light to the substrate where there are no thin film transistors and the storage capacitors.
In the top emission type organic electroluminescent display device according to the related art, the thin film transistors and the organic electroluminescent diodes are formed over the first substrate, and an additional second substrate is attached to the first substrate to encapsulate the organic electroluminescent device. However, when the thin film transistors and the organic electroluminescent diode are formed on the same substrate in this way, production yield of the organic electroluminescent display devices is determined by a multiplication of the thin film transistor's yield together with the organic electroluminescent diode's yield. Since the organic electroluminescent diode's yield is relatively low, the production yield of the overall organic electroluminescent display devices becomes limited. For example, even when thin film transistors are well fabricated, the organic electroluminescent display devices can be a poor product because of the defects of an organic electroluminescent layer that has about 1000 angstroms (Å) thickness. This results in loss of materials and increased production costs.